This invention relates generally to interferometer systems. This invention relates more particularly to a system and method for heterodyne interferometer compensation of a type of non-linearity occurring at high velocity.
Light leakage between beams in an interferometer in metrology produces measurement results that depart periodically from the idealxe2x80x94known as non-linearity.
One prior art technique for compensating for non-linearity in a homodyne interferometer involves balancing the two in-phase and quadrature signals within the interferometer for offset, gain and orthogonality. The signals are digitized, computed for imbalance, and analog electronics are used to inject offset and gain compensations. The compensation components are all contained within the interferometer itself. In a heterodyne interferometer, there are no in-phase and quadrature signals to balance. Thus, such a compensation method is not applicable to a heterodyne interferometer.
It would be desirable to provide a system and method for non-linearity compensation of interferometer position data using digital numerical processing, wherein the non-linearity is of a type that occurs at high velocities.
One form of the present invention provides a method for compensating non-linearity of the high-velocity type manifested in heterodyne interferometer position data. A plurality of groups of digital position values are received. A plurality of groups of digital phase values from a measurement channel are received. A first group of the digital position values and digital phase values are digitally processed to generate a plurality of block data values. The plurality of block data values are digitally processed to generate at least one quasi-static non-linearity parameter. A second group of the digital position values are compensated based on the at least one quasi-static non-linearity parameter.